1. Field of the Invention
The present invention relates to a hysteretic superconductor-insulator-superconductor (SIS) junction mixer and, more particularly, to a subharmonically pumped SIS junction mixer realized by not suppressing the critical current of the junction. The hysteretic mixer thus operates in a switching mode, providing effective low-noise mixing including conversion gain.
2. Description of the Prior Art
Subharmonically pumped mixing has been analyzed as early as 1976, as discussed in the article "Millimeter-Wave Receivers with Subharmonic Pump" by T. F. McMaster et al in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-24, Dec. 28, 1976 at pp. 948-952, and the related U.S. Pat. No. 4,000,469 issued to T. F. McMaster on Dec. 28, 1976. There, a frequency mixer-downconverter is dissolved which can be tuned as either a single sideband mixer or a double sideband mixer and which has a wide tunable RF bandwidth and low conversion loss. In the downconverter, a high frequency input signal is coupled from an input waveguide to a shielded suspended stripline, and two encapsulated Schottky barrier diodes are connected proximate to the waveguide-to-stripline transition between the inner conductor and the outer conducting channel of the stripline. The two diodes intermodulate the input signal with a subharmonic pumping signal which is also coupled to the suspended stripline from a separate waveguide input. An intermediate frequency signal, at a frequency equal to the difference between the input signal and twice the frequency of the pumping signal, is generated as an intermodulation product and transmitted on the suspended stripline to an output.
An alternative mixer arrangement is disclosed in U.S. Pat. No. 4,188,584 issued to H. Hoogstraate on Feb. 12, 1980 wherein the encapsulation surrounding the Schottky barrier diode intermodulator, as found in the hereinbefore mentioned McMaster arrangement, is removed, thereby increasing the bandwidth of the mixer over the prior art McMaster design.
The above-described mixers are built with an antiparallel Schottky barrier diode pair as the mixing element. A single Schottky barrier diode does not have the antisymmetric I-V characteristics neccessary for efficient subharmonically pumped mixing. In order to obtain an antisymmetric I-V characteristic, two matched Schottky barrier diodes are connected in anti-parallel.
Superconductor-insulator-superconductor (SIS) junctions do have naturally antisymmetric and highly nonlinear I-V characteristics as predicted by D. B. Josephson as early as 1962, where Josephson showed that a supercurrent would flow between two superconductors separated by a thin insulating barrier (i.e., a supercurrent tunnel junction). It is a characteristic of a superconducting tunnel junction which exhibits the Josephson effect, that the voltage acorss the junction remains zero over a range of supercurrents below a critical tunneling supercurrent. When the supercurrent flow through the junction exceeds the critical current, the voltage across the junction abruptly jumps to some higher value.
Further discovery in relation to these junction devices, as disclosed in U.S. Pat. No. 3,281,609 issued to J. M. Rowell on Oct. 25, 1966, found that as the junction current flow is reduced from above the critical supercurrent, a tunneling supercurrent less than the critical current is reestablished through the junction and the junction voltage again drops to zero, enabling these cryogenic junctions to function as switching and logic devices.
There are several mechanisms by which an SIS junction may be operated as a subharmonically pumped mixer. By suppressing the Josephson current with a magnetic field, as discussed by B. S. Deaver, Jr., in the article "Physics of Superconducting Devices", appearing in The Science and Technology of Superconductivity, W. D. Gregory, ed., Plenum Press, New York 1973, at page 559, an I-V characteristic very similar to the I-V characteristic for the antiparallel diode pair is obtained. Therefore, the analogous resistive mixing process is feasible.
The problem remaining in the prior art, then, is to achieve low-noise mixing with gain, utilizing the naturally antisymmetric SIS junction.